CBSE Class 10 Science 2017 · Paper 31/1/C · Question 11

Covalent compounds are compounds in which the atoms are held together by covalent bonds, that is, by the sharing of pairs of electrons between atoms — for example methane, ${\text{CH}}_4$, in which carbon shares one electron pair with each of four hydrogen atoms. Difference from ionic compounds: an ionic compound is formed by the transfer of electrons from a metal to a non-metal, which produces oppositely charged ions held together by strong electrostatic forces (as in sodium chloride). In a covalent compound the electrons are shared, no ions are formed, and compounds such as methane consist of separate molecules with only weak forces between them. So ionic compounds have high melting and boiling points and conduct electricity when molten or dissolved in water, while covalent compounds of this kind have low melting and boiling points and are generally poor conductors. Two properties of covalent compounds: 1. They generally have low melting and boiling points, because the forces between their molecules are weak (methane melts at $90$ K and boils at $111$ K). 2. They are generally poor conductors of electricity, because they contain no ions.